How to pass a reference to a void* from C++/CLI to a native C function - c++-cli

Question

I'm trying to call a native Windows API from managed C++/CLI. One of the arguments is a void**. The idea is that the function will allocate a memory structure and return a void pointer to the caller, which should be passed back to the API on the next call. So I need to allocate storage for a pointer on the managed side and pass a reference to the C API. I can't figure out how to do this.
I've tried declaring a void * in the caller and passing a reference via various operators: &, internal_ptr<>, pin_ptr<>. I did the same with an IntPtr. I get errors saying the compiler can't convert this to a void**.
Here's one attempt using IntPtr and pin_ptr. I get the following compile error on line 28 (the line that declares the pin_ptr):
E0144 a value of type "interior_ptr<System::IntPtr>" cannot be used to initialize an entity of type "cli::pin_ptr<void *>"
#include <msclr\marshal.h>
using namespace msclr::interop;
using namespace System;
namespace CLRStorage
{
public ref class CompoundFile
{
private:
String ^ pathname;
IntPtr pRootStorage;
public:
CompoundFile CompoundFile::Create(String^ path)
{
STGOPTIONS stgOptions;
stgOptions.usVersion = 1;
stgOptions.reserved = 0;
stgOptions.ulSectorSize = 4096;
stgOptions.pwcsTemplateFile = NULL;
auto cf = gcnew CompoundFile();
cf->pathname = path;
marshal_context^ context = gcnew marshal_context();
pin_ptr<void*> ppRootStorage = &cf->pRootStorage;
StgCreateStorageEx(
context->marshal_as<WCHAR*>(path),
STGM_READWRITE & STGM_CREATE,
STGFMT_DOCFILE,
0,
&stgOptions,
NULL,
IID_IStorage,
ppRootStorage);
}
};
}

Answer 1

IntPtr can be converted to and from void*, but it isn't the same type.
Since the parameter is out-only, the simple solution is just to use a temporary:
void* pRootStorage;
StgCreateStorageEx(
context->marshal_as<WCHAR*>(path),
STGM_READWRITE & STGM_CREATE,
STGFMT_DOCFILE,
0,
&stgOptions,
NULL,
IID_IStorage,
&pRootStorage);
cf->pRootStorage = IntPtr(pRootStorage);
This will actually be a tiny bit faster as well, because no pinning is needed.
You also have a separate problem with bad member function syntax. You want
static CompoundFile^ Create(String^ path)
instead of
CompoundFile CompoundFile::Create(String^ path)
and don't forget to
return cf;
Then, marshal_context is not a ref class, so this line is wrong:
marshal_context^ context = gcnew marshal_context();
Instead use
marshal_context context;
and since it is not a pointer,
context.marshal_as<WCHAR*>(path)